1. Field of the Invention
The present invention is directed to a computer tomography apparatus, and in particular to such an apparatus having a rotating frame surrounding a measuring region, with a radiation source and a row of detector elements being mounted on the frame.
2. Description of the Prior Art
Computer tomography devices are known in the art, such as that disclosed in German OS No. 2538517 corresponding to U.S. Pat. No. 4,002,917, which have a rotating frame on which a radiation source and a radiation detector are mounted. The radiation source generates a fan-shaped x-ray beam which penetrates an examination region in the interior of the frame, with radiation attenuated by a patient being incident on the radiation detector. The frame is rotatable around an axis extending through the measuring region, and disposed perpendicularly relative to the plane of the x-ray beam. The examination subject is thus transirradiated from different directions. A computer is provided which uses the data from the radiation detector to generate a cross-sectional image of the examination subject. This known device also has means for undertaking a periodic deflection of the focus of the x-ray source in the plane of the x-ray beam perpendicular to a center normal of the radiation detector
In a computer tomography apparatus of this type, the measured values are described by two parameters. The first parameter, .alpha., identifies the angular position of the focus centroid relative to a fixed axis. The second parameter, .beta., is the angle between the central ray of the x-ray beam and a connecting line between the focus and the detector element under consideration.
If the focus in the rotating frame has a fixed position, i.e., is not deflected, there are as many different .beta. values as detector elements. If the focus is periodically deflected on either side of a point on the center normal of the detector, in a direction perpendicular to the center normal, measured values for a plurality of .beta. values can be acquired with a single, defined detector element. This means the total number of measured values is increased, and thus the image resolution can also be increased.
If S(t) is the function defining the periodic movement of the focus with respect to time t, then S(t+T)=S(t) and S(t+T/2)=-S(t) are valid, where T is the period of the focus deflection. The focus centroid in the interval 0.ltoreq.t.ltoreq.T/2 is different from the focus centroid in the interval T/2.ltoreq.t.ltoreq.T.
Per detector element, a measured value in the first interval, and a further measured value in the second interval, can then be formed using integrators. Given a suitable selection of the amplitude of S(t), dependent on the shape on the function S, it can be achieved that the .beta. values for the second interval lie between the .beta. values of the first interval.
If all measured values in an interval are simultaneously formed, as in conventional devices, a memory must be provided in which the measured values available at the end of an interval are stored until derivation of a digital value proportional to the measured value. For example, a double integrator can be employed for each detector element for this purpose, the double integrator being switched at points in time n . T/2, with n being the number of measured data sets, and n=1, 2 . . .